Measuring Electron Recoil from Photon Emission: Experiments & Methods

[Danyon]

Has electron recoil due to photon emission ever been confirmed by experiment? cause I can't find any reference to electron recoil being measured anywhere I look. If it has been measured, what methods do they use?

 

[Orodruin]

What do you mean by photon emission by an electron? A free electron cannot emitt a photon so what kind of process are you imagining?

 

[Danyon]

What do you mean by photon emission by an electron? A free electron cannot emitt a photon so what kind of process are you imagining?

Any situation where an electron emits a photon, say in a free electron laser, or maybe after an electron spin flip. my question is, how can the recoil be measured?

 

[bhobba]

Any situation where an electron emits a photon, say in a free electron laser, or maybe after an electron spin flip. my question is, how can the recoil be measured?

QED does not have a picture of emission like that. Even ordinary QM doesn't - there is no such thing as recoil - the electron changes state - and a photon is detected - pictures like emitted, recoil etc are classical - QM is not classical eg electrons do not have a well defined position and momentum for the idea of recoil to even make sense.

Thanks
Bill

 

[Danyon]

QED does not have a picture of emission like that. Even ordinary QM doesn't - there is no such thing as recoil - the electron changes state - and a photon is detected - pictures like emitted, recoil etc are classical - QM is not classical eg electrons do not have a well defined position and momentum for the idea of recoil to even make sense.

Thanks
Bill

Alright, is there any way to detect the electrons state change? surely if the energy of the light emitted is high enough the recoil would be measurable .

 

[bhobba]

Alright, is there any way to detect the electrons state change? surely if the energy of the light emitted is high enough the recoil would be measurable .

Recoil is change in momentum. Electrons in atoms do not have a definite momentum for the concept to be applicable.

Thanks
Bill

 

[Danyon]

Recoil is change in momentum. Electrons in atoms do not have a definite momentum for the concept to be applicable.

Thanks
Bill

So an object emitting light won't experience recoil? What about in a free electron laser?

 

[bhobba]

What do you mean by photon emission by an electron? A free electron cannot emitt a photon so what kind of process are you imagining?

Just to elaborate a bit further recoil is change in momentum. For that to be meaningful the electron would need to be in a state of definite momentum which only happens for a free electron - which can't emit a photon. Its not a concept applicable to photon emission.

Thanks
Bill

 

[bhobba]

So an object emitting light won't experience recoil? What about in a free electron laser?

What about it? Do all electrons in a free electron laser gain and loose the same energy to make such a concept as individual electron recoil meaningful? As far as I know its a phenomena of a beam of electrons going through a magnetic field.

Before going any further define the exact set-up you are considering, what you are defining as recoil, and exactly how you will measure it.

Thanks
Bill

 

[Danyon]

What about it? Do all electrons in a free electron laser gain and loose the same energy to make such a concept as individual electron recoil meaningful?

Before going any further define the exact set-up you are considering, what you are defining as recoil, and exactly how you will measure it.

Thanks
Bill

When an electron emits a photon, conservation of momentum implies that the electron will recoil in response, this should happen in any situation where light is emitted, as for how I measure it, well that's the question I asked, I don't know how they measure electron recoil from photon emission

 

[Danyon]

Recoil is change in momentum. Electrons in atoms do not have a definite momentum for the concept to be applicable.

Thanks
Bill

when I shine light on my crookes radiometer it starts spinning, isn't this because the electrons in the atoms absorb the lights momentum?

 

[bhobba]

When an electron emits a photon, conservation of momentum implies that the electron will recoil in response,

That's false in QM. For it to even be meaningful the electron needs to have a definite momentum. It is a basic fact that only free electrons have a definite momentum.

Thanks
Bill

 

[bhobba]

when I shine light on my crookes radiometer it starts spinning, isn't this because the electrons in the atoms absorb the lights momentum?

So? Photons have momentum - that does not mean electrons recoil when they absorb it.

Thanks
Bill

 

[Danyon]

So? Photons have momentum - that does not mean electrons recoil when they absorb it.

Thanks
Bill

Actually they would have to recoil or else the crookes radiometer wouldn't spin.

 

[bhobba]

Actually they would have to recoil or else the crookes radiometer wouldn't spin.

That's not true. I suggest you look up something called Ehrenfest's Theorem. Its also relevant to a free electron laser.

Can I ask where you learned QM from? What I am saying is very very basic to QM.
.
Thanks
Bill

 

[Danyon]

That's not true. I suggest you look up something called Ehrenfest's Theorem.
.
Thanks
Bill

Ehrenfest's theorem states that the expectation values of quantum mechanics (QM) obey the classical laws of physics. This in turn means that the vast experimental evidence in support of classical mechanics do not contradict QM.

Regardless of that, the momentum that is absorbed by electrons in the atoms of the crookes radiometer does result in a recoil that is measurable, I think this means that the electrons do in fact recoil when they absorb the light, why not recoil when emitting light?

 

[jtbell]

when I shine light on my crookes radiometer it starts spinning, isn't this because the electrons in the atoms absorb the lights momentum?

No.

https://en.wikipedia.org/wiki/Crookes_radiometer#Explanations_for_the_force_on_the_vanes

 

[bhobba]

Regardless of that, the momentum that is absorbed by electrons in the atoms of the crookes radiometer does result in a recoil that is measurable, I think this means that the electrons do in fact recoil when they absorb the light, why not recoil when emitting light?

Your thinking is incorrect. That momentum change is a valid concept for a large ensemble of electrons does not imply it is a meaningful concept for individual electrons. That is very very basic foundational QM.

Again I ask you - where have you learned QM from? If you haven't I suggest you go and do some study on it.

Thanks
Bill

 

[bhobba]

No.
https://en.wikipedia.org/wiki/Crookes_radiometer#Explanations_for_the_force_on_the_vanes

You learn something new every day. I too have seen explanations that its caused by the pressure of light.

Thanks
Bill

 

[Danyon]

Your thinking is incorrect. That momentum change is a valid concept for a large ensemble of electrons does not imply it is a meaningful concept for individual electrons. That is very very basic foundational QM.

Again I ask you - where have you learned QM from? If you haven't I suggest you go and do some study on it.

Thanks
Bill

I should change my original question, I actually wanted to know if it was possible to measure the recoil of large number of electrons when they emitted light, say by measuring the recoil of a laser, i couldn't find any information online about that.

 

[bhobba]

I should change my original question, I actually wanted to know if it was possible to measure the recoil of large number of electrons when they emitted light, say by measuring the recoil of a laser, but i couldn't find any information online about that.

That I do not know the answer to, or even if in QED, which is more advanced than QM, its meaningful. I think - no - but others know more of QED than I do.

Thanks
Bill

 

[Danyon]

Would it then be possible to create a reactionless drive like effect just by firing a laser at the interior wall of a spaceship, If the laser doesn't recoil the lights momentum should slowly push the ship forward

 

[fzero]

I should change my original question, I actually wanted to know if it was possible to measure the recoil of large number of electrons when they emitted light, say by measuring the recoil of a laser, i couldn't find any information online about that.

The recoil on the laser would be around $10^{-8}$ Newtons per Watt of laser power. That's pretty small to directly measure, since you'd have to isolate all sorts of environmental vibrations that would hide the recoil. This this article describes measurements at LIGO of vibration of one of the mirrors due to radiation pressure from a 1/2 W laser.

 

[Nugatory]

Would it then be possible to create a reactionless drive like effect just by firing a laser at the interior wall of a spaceship, If the laser doesn't recoil the lights momentum should slowly push the ship forward

Not according to any generally accepted physics.

You and Bhobba have been talking past one another for many posts now. There's a difference between measuring the effects of momentum conservation in a system (such as a laser) in which light emission is happening, and observing electrons changing state by recoiling as they emit photons. It's very tempting to say that the former necessarily implies the latter, but QM doesn't work that way; wave functions apply to the system as whole, so there's no way of attributing the overall change in (the expectation value of the) momentum to changes in the constituent particles.